We will examine thyroid hormone effects on cell growth in cultured GC cells, a thyroid hormonesensitive cell line that produces growth hormone. Since we have synchronized the growth cycle of these cells, we now intend to study the effects of T3 on cellular metabolism at specific points of the cell growth cycle. Since the G1 period is most responsive to hormones and other growth factors, we will first determine the temporal kinetics of G1 and the effects of T3 on the duration of G1. We will then examine the role of T3 in the induction of specific proteins and phosphoproteins at the G1 commitment point. These are considered the most likely candidates for regulation of G1 and cell growth rate. Since we have already characterized the temporal pattern of nuclear T3 receptor concentration in relation to the cell cycle of synchronized GC cells, we will now determine whether the observed changes in receptor are due to changes in nuclear T3 receptor synthesis or degradation rates. We also propose to examine the physicochemical characteristics of nascent nuclear T3 receptor in conjunction with nascent DNA in the S phase of the cell cycle. We hope that these studies will elucidate in more intimate detail the nature of the physicochemical associaton of receptor to DNA in chromatin. Since growth hormone is specifically induced by T3 in this cell line, we intend to determine the GH production and secretory rates in different phases of the cell cycle in relation to changes in concentration of nuclear T3 receptor and chromatin. The mechanisms underlying changes in GH production will be investigated by measuring GH synthesis rates as well as the concentration of GH mRNA. Further studies will be done to characterize the interaction between epidermal growth factor (EGF) and T3 at the level of nuclear T3 receptor. EGF appears to antagonize T3 effects on cell growth and GH production. In preliminary studies, EGF seems to decrease nuclear T3 binding. We propose to determine the quantitative aspects of nuclear T3 receptor:T3:EGF interactions and relate these quantitatively to the observed biological events.